Albert Barnes Commentary 1 John 2:9

"He that saith he is in the light and hateth his brother, is in the darkness even until now." — 1 John 2:9 (ASV)

He that saith he is in the light. This means that he has true religion, or is a Christian. See 1 John 1:7.

And hateth his brother. The word brother seems here to refer to those who professed the same religion. The word is indeed sometimes used in a larger sense, but the reference here appears to be to that which is properly brotherly love among Christians. Compare to Lucke, in loc.

Is in darkness even unto now. That is, he cannot have true religion unless he has love to the brethren. The command to love one another was one of the most solemn and earnest which Christ ever enjoined (John 15:17); he made it the peculiar badge of discipleship, or that by which his followers were to be everywhere known (John 13:35); and it is, therefore, impossible to have any true religion without love to those who are sincerely and truly his followers. If a man does not have that, he is in deep darkness, whatever else he may have, on the whole subject of religion. Compare to See Barnes on 1 Thessalonians 4:9.